1. Field of the Invention
The present invention relates to a radio communication apparatus and method, and more particularly to a radio communication apparatus and method of a spatial multiplex transmission scheme based on OFDM.
2. Description of the Related Art
A method for distributing a transmit signal to a plurality of RF (Radio Frequency) units and simultaneously transmitting signals of the same frequency through a plurality of antennas has been proposed as a technique for enhancing the rate of radio communication (see, for example, U.S. Pat. Nos. 6,058,105 and 6,144,711). In this method, the signals of the same frequency are transmitted with different digital beams, and a receiving terminal receives multiplex signals transmitted through different routes, and separates them to decode them.
As a result, the rate of transmission can be enhanced in accordance with the number of multiplex signals, without widening the frequency bandwidth used for communication. Thus, the method can enhance the spectral efficiency and hence the throughput.
On the other hand, in a multipath channel in which a plurality of signals having different propagation delay times between transmission and receiving terminals are transmitted, waveform distortion due to ISI (Inter Symbol Interference) may be a great factor for degrading the quality of communication. A system using orthogonal frequency division multiplexing (OFDM) is known as a system capable of compensating waveform distortion due to ISI when it receives signals of different propagation delay times.
In OFDM transmission schemes, subcarriers have different channel responses. Accordingly, if the methods proposed in U.S. Pat. Nos. 6,058,105 and 6,144,711 are employed as OFDM transmission schemes, different digital beams are used for different subcarriers. In this case, in the prior art, digital beams that cause all subcarriers to provide high receiving power are assigned to only a particular signal, which inevitably increases the difference in characteristics between spatial multiplex signals. Therefore, signals of a high multi-level modulation number or of a high coding rate must be assigned as the spatial multiplex signals, and accordingly, RF units of high accuracy capable of both transmitting and receiving such signals are required.
As described above, in the conventional radio communication apparatus, a big difference is found in receiving power between multiplexed signals. This means that unless a modulation scheme or coding rate of a high transmission rate is used for signals of a high receiving power, the channel responses of the signals cannot sufficiently be utilized. Accordingly, it is necessary to use an accurate apparatus capable of transmitting and receiving signals of a high transmission rate. There is a demand for the reduction of the cost of the radio communication apparatus and the reduction of chip area of the integrated circuits incorporated in the apparatus.